The Controller Area Network (CAN) bus standard ISO 11898 is designed to allow devices to communicate with each other using a 2-wire bus. The ISO 11898 standard is incorporated herein by reference. The data signals on the bus are differential, so any common mode signals are ideally nullified. The standard is primarily applied to communications in vehicles, and examples of devices that may communicate over the bus include engine control units, power steering control units, air bag control units, audio system control units, power window control units, etc. The CAN bus standard may also be applied to industrial environments (e.g., robotic control units), entertainment environments (e.g., video game control units), and other environments.
The various control units typically generate parallel data, and the data is packaged in frames in accordance with a protocol and transmitted serially, as differential bit signals, on the bus. Collision and arbitration rules are specified by the standard.
The present invention only deals with the bus driver (a transmitter) in a CAN, which is typically coupled to a twisted wire pair.
FIG. 1 illustrates a prior art CAN bus driver 10 for a particular device receiving serial data on line 12. In one example, the bus driver 10 receives a logical 0 bit on line 12, and a gate driver 14 generates a low PGATE voltage for turning on a PMOS transistor 16 and generates a high NGATE voltage for turning on an NMOS transistor 18. Thus, Vcc is applied to the high side bus line 20, and system ground is applied to the low side bus line 22. The lines 20 and 22 are coupled to the twisted pair cable 24 (a bus) via optional reverse current-blocking diodes 26 and 28 and the bus terminals CANH and CANL. The voltage differential for a logical 1 bit should be greater than 1.5 volts. This is called a dominant state. For a logical 1 bit on line 12, both transistors 16 and 18 are turned off (a high impedance), and the 120 ohm termination resistors 30 and 32 return the differential voltage on the bus to 0 volts. This is called a recessive state.
Various devices would be coupled to the cable 24 and also include a bus driver similar to the driver 10.
The common mode voltage, which is equal to the average of the CANH and CANL terminal voltages, ideally remains constant during transitions from the recessive state to the dominant state and during transitions from the dominant state back to the recessive state. Fluctuations of the common mode voltage result in electromagnetic emissions (EME), which are undesirable in electronic systems.
During the transition from the recessive state to the dominant state, the PMOS transistor 16 should turn on at exactly the same time and at the same rate as the NMOS transistor 18 in order for the average of the CANH and CANL terminal voltages to remain approximately constant throughout the dominant state. Likewise, during the transition from the dominant state to the recessive state, the PMOS transistor 16 should turn off at exactly the same time and at the same rate as the NMOS transistor 18.
In practical electronic devices, it is very difficult to ensure that two different open drain FETs of different types (PFET vs NFET) turn on and off at exactly the same time and rate. If the two devices do not turn on or off at the same rate, large changes in the common mode voltage may arise during the transitions, resulting in EME. The CAN bus driver 10 is very susceptible to producing large common mode variations. This is because the two transistors 16 and 18 act as high impedance current sources when they are turning on and off, during which their gate to source (Vgs) voltage is low and their drain to source voltage (Vds) is high. Under this condition, the common mode load is the parallel output impedance of these two transistors (plus the parallel impedance of the CAN receivers that are on the CAN bus). This results in a high common mode loading impedance that can be several tens of kilohms. Under these conditions, a small fractional difference in the currents simultaneously conducted by the PMOS transistor 16 and the NMOS transistor 18 during the turn-on or turn-off transitions may result in a common mode voltage fluctuation of a volt or more. This is unacceptable for EME considerations in many systems.
What is needed is a CAN driver that is less affected by the unequal currents conducted by the main driver transistors during the transitions between the dominant and recessive states.